1. Field of the Invention
The present invention relates to a method of producing a high concentration polymer solution, and particularly to a method of producing a high concentration polymer solution that is used for forming a cellulose acylate film.
2. Description Related to the Prior Art
As a method for producing a polymer film, there are a melt extrusion method and a solution casting method. In the solution casting method, the polymer solution in which a polymer is dissolved in a solvent is cast from a casting die on a belt or a rotating drum or a belt. The polymer solution is thereafter dried to become a polymer film, and the polymer film is peeled from the belt or the drum.
When the polymer film is formed from the polymer solution in the solution casting method, the polymer film is more excellent in the physical property and optical property than the melt extrusion method. Therefore, the latter method is preferable for forming a film base used for a photo film. Further, the polymer film formed in the solution casting method is more excellent in optical isotropy and constant thickness and has smaller amount of dusts than in the melt extrusion method. The polymer film whose polymer is cellulose acylate, especially cellulose triacetate (hereinafter TAC) is used for optoelectronic products, such as a protective film for a polarizing filter, an optical retardation film, transparent conductive film, antireflection film with antiglare property and the like.
In order to effectively produce the polymer film in the solution casting method, the concentration of the polymer in the polymer solution (or dope) is preferably high. When the concentration is low, the large amount of solvent is necessary for preparing the polymer solution, and the time for drying the polymer solution on the belt or the drum is long, which causes the low productivity. Further, a gas (or vapor) of the solvent that is generated in drying the solution is liquidized for recovery and reuse. The costs therefor depend on the time from casting to peeling, and accordingly, becomes higher when the concentration of the polymer becomes smaller.
Accordingly, it is preferable to prepare a high concentration polymer solution having high concentration for forming the polymer film in the solution casting method. However, there are some problems for preparing the high concentration polymer solution. For example, when the polymer powders are dissolved in the solvent, then undissolved clumps of the polymer powders are generated, which prevents the uniform dissolution. In order to dissolve the polymer uniformly, a high speed stirring machine and an inline dispersing machine are used as described in the Japanese Patent Laid-Open Publication No. 2001-340735. Further, the Japanese Patent Laid-Open Publications No. 2000-256468 and 2001-206981 propose methods for dissolving the polymer powder under high pressure and high temperature to improve the solubility and shorten the dissolving time and for dissolving the polymer powder under supercritical state. Further, the polymer may be dispersed and swollen and the temperature may become lower, to dissolve the polymer in the solvent effectively. However, when the concentration of the polymer solution becomes higher, the solubility becomes lower in any methods. The solubility in this dissolving method for dissolving a large amount of polymer depends on the concentration of the polymer solution.
Accordingly, a concentrating method is proposed, in which a low concentration polymer solution is concentrated to the high concentration polymer solution. In the concentrating method, as described in Japanese Patent Laid-Open Publication No. 4-259511, a thin film is formed of the polymer solution with rotating of rotating blades, and the polymer solution is evaporated such that the evaporated gas may be condensed to reuse the condensed solvent and the concentration of the polymer solution may become higher. Furthermore, the Japanese Patent Laid-Open Publication No. 2002-103357 discloses a splash concentration method, or a method in which the polymer solution having high temperature and high pressure is splashed to make evaporation of the solvent. The evaporated gas is separated from the polymer solution and the concentration of the polymer solution becomes higher in the flash concentrating method. The flash point method in the publication No. 2002-103357, as the structure of equipment is simple, has more merits in cost of equipment and running cost than the former method in the publication No. H4-259511.
However, in the flash concentration method of the prior art, as the polymer solution is splashed in a gas phase in the flash tank, part thereof is adhered on an inner surface of a flash tank. The inner surface of the flash tank is heated to supply the heat energy for the polymer solution. Accordingly, the skinning of gel-like material is often formed on the inner surface of the flash tank, and mixed into a condensed polymer solution, which makes the quality of the polymer film lower.